CONCORD, N.C. — Dale Earnhardt Jr. knew he had suffered a concussion in an August crash so jolting that other drivers tweeted about it immediately after the impact.

Earnhardt was too stubborn to see a doctor about it. He was too worried he would be yanked from his car, derailing his long-suffering Sprint Cup Championship hopes.

So he kept it a secret until a 25-car accident on the last lap Sunday at Talladega left him with a lingering headache.

NASCAR’s most popular driver sought medical attention from a neurosurgeon, who found Earnhardt had indeed suffered two concussions in six weeks and could not be medically cleared to race. Earnhardt said Thursday he will sit out the next two weeks, at Charlotte and Kansas, ending his championship chances.

“I would love to race this weekend, and I feel perfectly normal and feel like I could compete if I were allowed to compete,” Earnhardt said. “But I think that the basis of this whole deal is that I’ve had two concussions in the last (six) weeks, and you can’t layer concussions. It gets extremely dangerous.”

A decade ago, it was Earnhardt who helped spur changes in how NASCAR handled drivers showing signs of a concussion.

He self-diagnosed a concussion from an accident at California, but didn’t tell anyone about it until revealing in an interview weeks later that he’d been having difficulty focusing and communicating with his crew chief. Within days of his admission, NASCAR strengthened its commitment to keeping drivers with concussions off the track.

NASCAR ruled that drivers unable to drive their car back to the garage after an accident had to make a mandatory trip to the infield care center. The attending physician could then refer a driver to a neurosurgeon for a CT scan or MRI if they suspected a concussion.

Clearance to race after suffering a concussion is not given until after a driver obtains a medical release.

“I think we’ve got a pretty good history of sending drivers to the care center and then also to a neurologist if we think there may be any cause to do so,” said NASCAR senior vice president Steve O’Donnell, who added that only nine drivers from NASCAR’s three national series have suffered concussions in the last five years.

Of course, Earnhardt proved Thursday that NASCAR isn’t always in the know.

Earnhardt’s first concussion this season came in an Aug. 29 wreck during a tire test at Kansas. His crash into the wall when his right front tire failed was so hard that Brad Keselowski immediately tweeted about. Earnhardt was seen after the accident in the back of an ambulance, but was not treated in the infield care center and did not seek further examination elsewhere.

He attended a Washington Redskins exhibition game later that night, but admitted Thursday he knew he suffered a concussion.

“You know your body, and you know how your mind works, and I knew something was just not quite right,” he said. “But I decided to just try to push through and work through it. I’d had concussions before and knew exactly kind of what I was dealing with.”

Kansas Speedway president Pat Warren said Thursday the speedway was well-staffed with medical personnel for the August tire test, and that Earnhardt was assessed in the ambulance by a paramedic who has worked for 12 years at the track.

“She went up and assessed Dale with the questions they ask when they are worried about a head injury — he answered them all correctly,” Warren said.

Warren said he also spoke to Earnhardt after he was back in the garage and already changed out of his firesuit, and the driver said he felt fine. The fire chief on staff at the track that day also asked Earnhardt in the garage the same set of questions intended to determine head injuries and checked for dilated pupils, Warren said.

Earnhardt said he regrets not seeing anyone about that concussion, but admitted to crew chief Steve Letarte what had happened and said he would not have raced at Atlanta the next weekend if he’d not felt well.

“With the Chase coming up, I didn’t know how difficult — if I was to volunteer myself to get medical attention and be removed from the car, I didn’t know how difficult it would be to get back in,” he said. “But I was honest with Steve and told Steve, ‘When we get to Atlanta and if I don’t feel good, I’m going to be honest with you and tell you that we need to have something as a backup plan for me to get out of the car.”’

He didn’t give himself the same leeway this week, when he suffered a lingering headache following the last lap accident at Talladega. Although Earnhardt said the impact was half as hard as the Kansas hit, “I knew as soon as it happened that I had reinjured myself.”

Earnhardt didn’t have to make the mandatory trip to the care center on Sunday because he was able to drive his car away from the accident — teammate Jimmie Johnson even caught a lift on the window back to the garage.

He sought out personal physician Dr. Jerry Petty, a neurosurgeon who consults with NASCAR and the NFL’s Carolina Panthers, on Tuesday and underwent an MRI on Wednesday, his 38th birthday. The MRI showed no damage, but Petty said Earnhardt was candid about his symptoms that last six weeks and he diagnosed a pair of concussions.

“His eyes did what they were supposed to do, his balance tests and so forth are perfect,” Petty said. “The one test, the one symptom that is more important than all the tests is headache, and as long as there’s any headache, the brain is not healed.”

Petty said Earnhardt will not be cleared to race again until he’s gone at least four days headache-free, and goes through a test in which Petty tries to provoke a headache.

“If we can’t, we’ll let him go out and drive a lap or two and see how that goes, and if that goes well, we’ll probably clear him to race,” Petty said.

Hendrick Motorsports tabbed Regan Smith to replace Earnhardt in the No. 88 Chevrolet the next two races. Smith had been scheduled to drive the No. 51 for Phoenix Racing in Saturday night’s race at Charlotte Motor Speedway, and AJ Allmendinger will now drive that car in his first start since his July 7 suspension for failing a random drug test.

This weekend’s race will be the first since Sept. 3, 1979, that will not include an Earnhardt in the field. Earnhardt’s father, seven-time NASCAR champion Dale Earnhardt, was killed on the last lap of the 2001 Daytona 500.

Earnhardt, who snapped a 143-race winless streak this season that dated to 2008, opened the Chase as a strong contender to win his first Sprint Cup title. This injury ends his chances, which were a longshot anyway because of a mediocre start to the Chase.

The crash at Talladega dropped him to 11th in the Chase standings, and missing two races means he’ll most certainly finish last in the 12-driver Chase race. He’ll also end his streak of 461 consecutive starts, which is the fifth longest active streak in the Sprint Cup Series.

“I think a lot of guys would try to play hurt,” Hendrick said. “I applaud Dale for raising his hand and going in there and getting checked out.”

O’Donnell said NASCAR will likely review the Kansas accident and see if series officials should have handled Earnhardt’s care differently.

Other sports have struggled with their handling of concussion, especially football and hockey.

After years of denying any connection between head injuries suffered on the field and later brain injuries, the NFL now has a protocol before players can return if they show symptoms of a concussion. The NFL is also being sued by more than 3,000 former players who claim it didn’t do enough to protect them.

O’Donnell said drivers have to take some responsibility for their health, as well.

“The process of an evaluation for any athlete or driver, it’s not just NASCAR making the call,” O’Donnell said. “It has to be the driver as well letting us know how he’s feeling. We’ll look at Kansas and see what we may be able to do better.”

Nationwide Series driver Eric McClure was held out six weeks this season with lingering effects of a concussion suffered at Talladega. McClure said the concussion he sustained in the May 5 race was the third of his career, and the frequency was one of the main reasons his doctors and NASCAR officials made him sit out for an extended period of time.

Sprint Cup Series points leader Keselowski knows how hard it is for a driver to consider sitting out a race. He broke his ankle in a crash while testing last summer, and although he had a backup driver on standby, he didn’t miss a Cup start.

“It’s your worst fear, to not be in the race,” he said. “Missing the show is terrible.”